Gridiron Pendulum

Gridiron Pendulum is a special form of pendulum constructed so that its period of oscillation shall be independent of variations in temperature. This period depends on the distance of the centre of gravity of the pendulum from its point of support, and in the ordinary type of pendulum this is increased by expansion of the pendulum rod when the temperature rises, the effect being to make the pendulum oscillate more slowly. In the gridiron form, which is due to Harrison, the downward expansion of the central bar, which may be of iron, is compensated by the upward expansion of two lateral bars of another metal such as zinc, whose expansion coefficient is greater. These are supported on a cross-piece fixed transversely at the lower end of the central bar. These lateral pieces support another cross-piece at their upper extremity which may again carry a pair of iron bars from which the pendulum bob hangs. If the upward expansion of the zinc be not sufficient to balance the downward expansion of the iron, further addition of pairs of bars of zinc and iron must be added till the required ratio between the length of iron and of zinc be obtained. The mercurial pendulum (q.v.), due to Graham, is a much more compact arrangement for compensation on the same principle.